Postgres Sink: make `__key` and `timestamp` columns optional

[francoishornoy]

Is your feature request related to a problem? Please describe.
We would like to not have the compulsory columns __key and timestamp (or at least, not with these names).

Describe the solution you'd like
Users who need a __key and timestamp columns can easily add them into the value dict through sdf.apply() so there's no reason to enforce these 2 columns. But given it's already out there, in order to keep backward-compatibility, maybe a metadata: bool = True arg can be added to the constructor to enable/disable saving these 2 columns.

@gwaramadze @tim-quix @tomas-quix FYR and thanks for this Sink!